BACKGROUND: Standard screening mammogram views focus on the lateral and posterior breast tissues; the view that best visualizes the intermammary area, the 90-degree lateral or cleavage view, is not typically performed. The limited field of view of mammogram increases the importance of additional imaging with CT, ultrasound, and MRI as a tool in diagnosing neoplasm in the intermammary area. To date, intermammary primary breast carcinoma has been reported only a few times in the literature, and none of these described direct sternal invasion at presentation.

CASE REPORT: An 85-year-old woman with a past medical history of follicular non-Hodgkin lymphoma treated with bendamustine-rituximab in 2017 (without chest radiation) and significant family history of breast cancer presented to our clinic 2 months after noticing a hard, non-tender nodule on her sternum. She had undergone a routine screening mammogram 8 months prior, which was unremarkable. Subsequent diagnostic workup including targeted ultrasound, and PET-CT identified a hypermetabolic presternal mass. Notably, diagnostic mammography 8 months after the initial screening mammogram also failed to properly visualize the lesion. Biopsy confirmed ER+/PR+, HER2- adenocarcinoma consistent with breast origin. Despite negative initial staging for distant metastasis, pathological examination after resection revealed direct invasion into the sternum, necessitating partial sternectomy.

CONCLUSIONS: This case highlights a rare intermammary presentation of breast carcinoma with direct sternal invasion, a feature not previously reported. It emphasizes the blind spot of standard screening mammography in the medial breast/cleavage area and the necessity of targeted imaging for palpable parasternal masses, even in the setting of recent negative screens.

Keywords: Adenocarcinoma, Chemotherapy, Adjuvant, Mammography, Breast Neoplasms, Mammography, Sternum, Adenocarcinoma, Aromatase Inhibitors, Case Reports